Photovoltaic devices and method of making

ABSTRACT

Embodiments of a photovoltaic device are provided herein. The photovoltaic device can include a layer stack and an absorber layer disposed on the layer stack. The absorber layer can include a first region and a second region. Each of the first region of the absorber layer and the second region of the absorber layer can include a compound comprising cadmium, selenium, and tellurium. An atomic concentration of selenium can vary across the absorber layer. The first region of the absorber layer can have a thickness between 100 nanometers to 3000 nanometers. The second region of the absorber layer can have a thickness between 100 nanometers to 3000 nanometers. A ratio of an average atomic concentration of selenium in the first region of the absorber layer to an average atomic concentration of selenium in the second region of the absorber layer can be greater than 10.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/032,531, filed on Jul. 11, 2018, now U.S. Pat. No.10,141,473, which is a continuation of U.S. patent application Ser. No.13/912,782, filed on Jun. 7, 2013, now U.S. Pat. No. 10,062,800, each ofwhich is incorporated by reference herein, in its entirety, for allpurposes.

BACKGROUND

The invention generally relates to photovoltaic devices. Moreparticularly, the invention relates to photovoltaic devices includingselenium, and methods of making the photovoltaic devices.

Thin film solar cells or photovoltaic (PV) devices typically include aplurality of semiconductor layers disposed on a transparent substrate,wherein one layer serves as a window layer and a second layer serves asan absorber layer. The window layer allows the penetration of solarradiation to the absorber layer, where the optical energy is convertedto usable electrical energy. The window layer further functions to forma heterojunction (p-n junction) in combination with an absorber layer.Cadmium telluride/cadmium sulfide (CdTe/CdS) heterojunction-basedphotovoltaic cells are one such example of thin film solar cells, whereCdS functions as the window layer.

However, thin film solar cells may have low conversion efficiencies.Thus, one of the main focuses in the field of photovoltaic devices isthe improvement of conversion efficiency. Absorption of light by thewindow layer may be one of the phenomena limiting the conversionefficiency of a PV device. Further, a lattice mismatch between thewindow layer and absorber layer (e.g., CdS/CdTe) layer may lead to highdefect density at the interface, which may further lead to shorterinterface carrier lifetime. Thus, it is desirable to keep the windowlayer as thin as possible to help reduce optical losses by absorption.However, for most of the thin-film PV devices, if the window layer istoo thin, a loss in performance can be observed due to low open circuitvoltage (V_(OC)) and fill factor (FF).

Thus, there is a need for improved thin film photovoltaic devicesconfigurations, and methods of manufacturing these.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the present invention are included to meet these andother needs. One embodiment is a photovoltaic device. The photovoltaicdevice includes a layer stack; and an absorber layer is disposed on thelayer stack. The absorber layer includes selenium, and an atomicconcentration of selenium varies non-linearly across a thickness of theabsorber layer.

One embodiment is a photovoltaic device. The photovoltaic deviceincludes a layer stack including a transparent conductive oxide layerdisposed on a support, a buffer layer disposed on the transparentconductive oxide layer, and a window layer disposed on the buffer layer.The layer stack further includes an absorber layer disposed on the layerstack, wherein the absorber layer includes selenium, and an atomicconcentration of selenium varies non-linearly across a thickness of theabsorber layer.

One embodiment is a method of making a photovoltaic device. The methodincludes providing an absorber layer on a layer stack, wherein theabsorber layer includes selenium, and wherein an atomic concentration ofselenium varies non-linearly across a thickness of the absorber layer.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings,wherein:

FIG. 1 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 2 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 3 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 4 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 5 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 6 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 7 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 8 is a schematic of a method of making a photovoltaic device,according to some embodiments of the invention.

FIG. 9 shows the Se concentration as a function of depth, in accordancewith one embodiment of the invention.

FIG. 10 shows the log-log plot of Se concentration as a function ofdepth, in accordance with one embodiment of the invention.

FIG. 11 shows the Se concentration as a function of depth, in accordancewith some embodiments of the invention.

DETAILED DESCRIPTION

As discussed in detail below, some of the embodiments of the inventioninclude photovoltaic devices including selenium.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about”, and “substantially” is not to be limited tothe precise value specified. In some instances, the approximatinglanguage may correspond to the precision of an instrument for measuringthe value. Here and throughout the specification and claims, rangelimitations may be combined and/or interchanged, such ranges areidentified and include all the sub-ranges contained therein unlesscontext or language indicates otherwise.

In the following specification and the claims, the singular forms “a”,“an” and “the” include plural referents unless the context clearlydictates otherwise. As used herein, the term “or” is not meant to beexclusive and refers to at least one of the referenced components (forexample, a layer) being present and includes instances in which acombination of the referenced components may be present, unless thecontext clearly dictates otherwise.

The terms “transparent region” and “transparent layer” as used herein,refer to a region or a layer that allows an average transmission of atleast 70% of incident electromagnetic radiation having a wavelength in arange from about 350 nm to about 1000 nm.

As used herein, the term “layer” refers to a material disposed on atleast a portion of an underlying surface in a continuous ordiscontinuous manner. Further, the term “layer” does not necessarilymean a uniform thickness of the disposed material, and the disposedmaterial may have a uniform or a variable thickness. Furthermore, theterm “a layer” as used herein refers to a single layer or a plurality ofsub-layers, unless the context clearly dictates otherwise.

As used herein, the term “disposed on” refers to layers disposeddirectly in contact with each other or indirectly by having interveninglayers therebetween, unless otherwise specifically indicated. The term“adjacent” as used herein means that the two layers are disposedcontiguously and are in direct contact with each other.

In the present disclosure, when a layer is being described as “on”another layer or substrate, it is to be understood that the layers caneither be directly contacting each other or have one (or more) layer orfeature between the layers. Further, the term “on” describes therelative position of the layers to each other and does not necessarilymean “on top of” since the relative position above or below depends uponthe orientation of the device to the viewer. Moreover, the use of “top,”“bottom,” “above,” “below,” and variations of these terms is made forconvenience, and does not require any particular orientation of thecomponents unless otherwise stated.

As discussed in detail below, some embodiments of the invention aredirected to a photovoltaic device including selenium. A photovoltaicdevice 100, according to some embodiments of the invention, isillustrated in FIGS. 1-5. As shown in FIGS. 1-5, the photovoltaic device100 includes a layer stack 110 and an absorber layer 120 disposed on thelayer stack 110. The absorber layer 120 includes selenium, and an atomicconcentration of selenium varies non-linearly across a thickness of theabsorber layer 120.

The term “atomic concentration” as used in this context herein refers tothe average number of selenium atoms per unit volume of the absorberlayer. The terms “atomic concentration” and “concentration” are usedherein interchangeably throughout the text. The term “variesnon-linearly across the thickness” as used herein means that therate-of-change in concentration itself varies across the thickness ofthe absorber layer 120.

As used herein the term “linear gradient” refers to the first derivativeof a given property, which when measured respect to a dimensionalparameter, such as the distance from the front contact is bothcontinuous and constant. For example, a step-wise distribution with afixed concentration of selenium (Se) at the front contact, which thenabruptly transitions to a different concentration after some distanceaway from the front contact, is non-linear due to the fact that thefirst derivative is non-continuous at the point where the concentrationof Se transitions from one value to another. An exponentially varyingdistribution is another example of a non-linear distribution since thevalue of the first derivative continuously changes as a function ofdistance. The linearity of a given distribution may be readily assessedby plotting the logarithm of the measured property versus the logarithmof the dimensional parameter. A linear gradient implies that the datawhen plotted this manner can be fit to a line with a unity slope. Asuper-linear distribution will have a slope greater than unity and asub-linear distribution will have a slope less than 1.

Measurement of a first derivative of a material property in a realmaterial implies averaging of the material property over a defineddimension and length scale, since the atomic nature of real materialsmay lead to local discontinuities of the first derivative. Thenon-linear distributions of interest according to some embodiments ofthe invention are in the axis that goes from the front contact to theback contact, which will be referred to as the z-axis or z-dimension.Thus, to measure the non-linearity of the distribution of a propertyalong the z-axis, it may be useful to average the measured propertiesover the orthogonal axes, x, y in order to minimize the effect ofgrain-boundaries and other local inhomogeneities on the measurement.

A lower limit for the averaging window is set by the polaron radius ofthe material which scales the typical “size” of a charge carrier withina real material:

$r_{p} = \sqrt{\frac{h}{4\pi\; m\;\omega}}$where h is Planck's constant, m is the effective mass of the chargecarrier, and w is the highest angular frequency of a typical vibrationof the lattice, which is typically an optical phonon. In cadmiumtelluride (CdTe), the effective mass of the electron is about 0.1 m_(e),where m_(e) is the mass of an electron in free space and the phononangular frequency is about 2.1×10¹³. Thus, the calculated polaron radiusis about 5 nm and a calculated polaron diameter is about 10 nm. Sinceproto-typical Gaussian or exponential wave functions have significantamplitude about 2-3 times their nominal characteristic size, then anestimate of the ‘size’ of charge carrier in CdTe based material is about30 nm. A typical charge carrier in a CdTe type material will sample a 30nm diameter sphere at any given time, and its behavior will to a largeextent be determined by the average physical properties within thissphere. Thus, to determine the degree of non-linearity relevant to theperformance of the photovoltaic cells in accordance with someembodiments of the invention, it may not be necessary to resolvenon-linearities in the distribution of a given property or compositionbelow a length scale of about 30 nm. An upper limit on the averagingrequired is set by the need to sample a sufficient number of points,i.e. 3, along the z axis so that the linearity of the distribution maybe determined.

In some embodiments, there is a step-change in the concentration ofselenium across the thickness of the absorber layer. In such instances,the selenium concentration may remain substantially constant for someportion of the thickness. The term “substantially constant” as used inthis context means that the change in concentration is less than 5percent across that portion of the thickness.

In some embodiments, the concentration of selenium varies continuouslyacross the thickness of the absorber layer 120. Further, in suchinstances, the variation in the selenium concentration may be monotonicor non-monotonic. In certain embodiments, the concentration of seleniumvaries non-monotonically across a thickness of the absorber layer. Insome instances, the rate-of-change in concentration may itself varythrough the thickness, for example, increasing in some regions of thethickness, and decreasing in other regions of the thickness. A suitableselenium profile may include any higher order non-linear profile.Non-limiting examples of suitable selenium profiles include anexponential profile, a top-hat profile, a step-change profile, asquare-wave profile, a power law profile (with exponent greater than 1or less than 1), or combinations thereof. FIG. 11 illustrates a fewexamples of representative non-linear selenium profiles in the absorberlayer 120. As will be appreciated by one of ordinary skill in the art,the profile of the selenium concentration may further vary after theprocessing steps, and the final device may include a diffused version ofthe profiles discussed here.

In some embodiments, the selenium concentration decreases across thethickness of the absorber layer 120, in a direction away from the layerstack 110. In some embodiments, the selenium concentration monotonicallydecreases across the thickness of the absorber layer 120, in a directionaway from the layer stack 110. In some embodiments, the seleniumconcentration continuously decreases across a certain portion of theabsorber layer 120 thickness, and is further substantially constant insome other portion of the absorber layer 120 thickness.

In certain embodiments, the absorber layer 120 includes a varyingconcentration of selenium such that there is lower concentration ofselenium near the front interface (interface closer to the frontcontact) relative to the back interface (interface closer to the backcontact). In certain embodiments, the absorber layer 120 includes avarying concentration of selenium such that there is higherconcentration of selenium near the front interface (interface closer tothe front contact) relative to the back interface (interface closer tothe back contact).

In certain embodiments, the band gap in the absorber layer 120 may varyacross a thickness of the absorber layer 120. In some embodiments, theconcentration of selenium may vary across the thickness of the absorberlayer 120 such that the band gap near the front interface is lower thanthe band gap near the back interface.

Without being bound by any theory, it is believed that a higherconcentration of selenium near the front interface relative to the backinterface may further allow for a higher fraction of incident radiationto be absorbed in the absorber layer 120. Moreover, selenium may improvethe passivation of grain boundaries and interfaces, which can be seenthrough higher bulk lifetime and reduced surface recombination. Further,the lower band gap material near the front interface may enhanceefficiency through photon confinement.

In some embodiments, the photovoltaic device 100 is substantially freeof a cadmium sulfide layer. The term “substantially free of a cadmiumsulfide layer” as used herein means that a percentage coverage of thecadmium sulfide layer (if present) on the underlying layer (for example,the interlayer or the buffer layer) is less than 20 percent. In someembodiments, the percentage coverage is in a range from about 0 percentto about 10 percent. In some embodiments, the percentage coverage is ina range from about 0 percent to about 5 percent. In certain embodiments,the photovoltaic device is completely free of the cadmium sulfide layer.

In certain embodiments, the absorber layer 120 may include aheterojunction. As used herein, a heterojunction is a semiconductorjunction that is composed of layers/regions of dissimilar semiconductormaterial. These materials usually have non-equal band gaps. As anexample, a heterojunction can be formed by contact between a layer orregion having an excess electron concentration with a layer or regionhaving an excess of hole concentration e.g., a “p-n” junction.

As will be appreciated by one of ordinary skill in the art, by varyingthe concentration of selenium in the absorber layer 120, a particularregion of the absorber layer 120 may be rendered n-type and anotherregion of the absorber layer 120 may be rendered p-type. In certainembodiments, the absorber layer 120 includes a “p-n” junction. The “p-n”junction may be formed between a plurality of regions of the absorberlayer 120 having different band gaps. Without being bound by any theory,it is believed that the variation in selenium concentration may allowfor a p-n junction within the absorber layer 120 or formation of ajunction between the absorber layer and the underlying TCO layer.

In some embodiments, the photovoltaic device may further include awindow layer (including a material such as CdS). In some embodiments,the absorber layer 120 may form a p-n junction with the underlyingbuffer layer or the window layer. As described earlier, the thickness ofthe window layer (including a material such as CdS) is typically desiredto be minimized in a photovoltaic device to achieve high efficiency.With the presence of the varying concentration of selenium in theabsorber layer, the thickness of the window layer (e.g., CdS layer) maybe reduced or the window layer may be eliminated, to improve theperformance of the present device. Moreover, the present device mayachieve a reduction in cost of production because of the use of loweramounts of CdS or elimination of CdS.

In some embodiments, as indicated in FIG. 2, the absorber layer 120includes a first region 122 and a second region 124. As illustrated inFIG. 2, the first region 122 is disposed proximate to the layer stack110 relative to the second region 124. In some embodiments, an averageatomic concentration of selenium in the first region 122 is greater thanan average atomic concentration of selenium in the second region 124.

In some embodiments, the selenium concentration in the first region 122,the second region 124, or both the regions may further vary across thethickness of the respective regions. In some embodiments, the seleniumconcentration in the first region 122, the second region 124, or boththe regions may continuously change across the thickness of therespective regions. As noted earlier, in some instances, therate-of-rate-of-change in concentration may itself vary through thefirst region 122, the second region 124, or both the regions, forexample, increasing in some portions, and decreasing in other portions.

In some embodiments, the selenium concentration in the first region 122,the second region 124, or both the regions may be substantially constantacross the thickness of the respective regions. In some otherembodiments, the selenium concentration may be substantially constant inat least a portion of the first region 122, the second region 124, orboth the regions. The term “substantially constant” as used in thiscontext means that the change in concentration is less than 5 percentacross that portion or region.

The absorber layer 120 may be further characterized by the concentrationof selenium present in the first region 122 relative to the secondregion 124. In some embodiments, a ratio of the average atomicconcentration of selenium in the first region 122 to the average atomicconcentration of selenium in the second region 124 is greater than about2. In some embodiments, a ratio of the average atomic concentration ofselenium in the first region 122 to the average atomic concentration ofselenium in the second region 124 is greater than about 5. In someembodiments, a ratio of the average atomic concentration of selenium inthe first region 122 to the average atomic concentration of selenium inthe second region 124 is greater than about 10.

The first region 122 and the second region 124 may be furthercharacterized by their thickness. In some embodiments, the first region122 has a thickness in a range from about 1 nanometer to about 5000nanometers. In some embodiments, the first region 122 has a thickness ina range from about 100 nanometers to about 3000 nanometers. In someembodiments, the first region 122 has a thickness in a range from about200 nanometers to about 1500 nanometers. In some embodiments, the secondregion 124 has a thickness in a range from about 1 nanometer to about5000 nanometers. In some embodiments, the second region 124 has athickness in a range from about 100 nanometers to about 3000 nanometers.In some embodiments, the second region 124 has a thickness in a rangefrom about 200 nanometers to about 1500 nanometers.

Referring again to FIG. 2, in some embodiments, the first region 122 hasa band gap that is lower than a band gap of the second region 124. Insuch instances, the concentration of selenium in the first region 122relative to the second region 124 may be in a range such that the bandgap of the first region 122 is lower than the band gap of the secondregion 124.

The absorber layer 120 also includes a plurality of grains separated bygrain boundaries. In some embodiments, an atomic concentration ofselenium in the grain boundaries is higher than the atomic concentrationof selenium in the grains.

Selenium may be present in the absorber layer 120, in its elementalform, as a dopant, as a compound, or combinations thereof. In certainembodiments, at least a portion of selenium is present in the absorberlayer in the form of a compound. The term “compound”, as used herein,refers to a macroscopically homogeneous material (substance) consistingof atoms or ions of two or more different elements in definiteproportions, and at definite lattice positions. For example, cadmium,tellurium, and selenium have defined lattice positions in the crystalstructure of a cadmium selenide telluride compound, in contrast, forexample, to selenium-doped cadmium telluride, where selenium may be adopant that is substitutionally inserted on cadmium sites, and not apart of the compound lattice

In some embodiments, at least a portion of selenium is present in theabsorber layer 120 in the form of a ternary compound, a quaternarycompound, or combinations thereof. In some embodiments, the absorberlayer 120 may further include cadmium and tellurium. In certainembodiments, at least a portion of selenium is present in the absorberlayer in the form of a compound having a formula CdSe_(x)Te_(1-x),wherein x is a number greater than 0 and less than 1. In someembodiments, x is in a range from about 0.01 to about 0.99, and thevalue of “x’ varies across the thickness of the absorber layer 120.

In some embodiments, the absorber layer 120 may further include sulfur.In such instances, at least a portion of the selenium is present in theabsorber layer 120 in the form of a quaternary compound includingcadmium, tellurium, sulfur, and selenium. Further, as noted earlier, insuch instances, the concentration of selenium may vary across athickness of the absorber layer 120.

The absorber layer 120 may be further characterized by the amount ofselenium present. In some embodiments, an average atomic concentrationof selenium in the absorber layer 120 is in a range from about 0.001atomic percent to about 40 atomic percent of the absorber layer 120. Insome embodiments, an average atomic concentration of selenium in theabsorber layer 120 is in a range from about 0.01 atomic percent to about25 atomic percent of the absorber layer 120. In some embodiments, anaverage atomic concentration of selenium in the absorber layer 120 is ina range from about 0.1 atomic percent to about 20 atomic percent of theabsorber layer 120.

As noted, the absorber layer 120 is a component of a photovoltaic device100. In some embodiments, the photovoltaic device 100 includes a“superstrate” configuration of layers. Referring now to FIGS. 3-6, insuch embodiments, the layer stack 110 further includes a support 111,and a transparent conductive oxide layer 112 (sometimes referred to inthe art as a front contact layer) is disposed on the support 111. Asfurther illustrated in FIGS. 3-6, in such embodiments, the solarradiation 10 enters from the support 111, and after passing through thetransparent conductive oxide layer 112, the buffer layer 113, andoptional intervening layers (for example, interlayer 114 and windowlayer 115) enters the absorber layer 120. The conversion ofelectromagnetic energy of incident light (for instance, sunlight) toelectron-hole pairs (that is, to free electrical charge) occursprimarily in the absorber layer 120.

In some embodiments, the support 111 is transparent over the range ofwavelengths for which transmission through the support 111 is desired.In one embodiment, the support 111 may be transparent to visible lighthaving a wavelength in a range from about 400 nm to about 1000 nm. Insome embodiments, the support 111 includes a material capable ofwithstanding heat treatment temperatures greater than about 600° C.,such as, for example, silica or borosilicate glass. In some otherembodiments, the support 111 includes a material that has a softeningtemperature lower than 600° C., such as, for example, soda-lime glass ora polyimide. In some embodiments certain other layers may be disposedbetween the transparent conductive oxide layer 112 and the support 111,such as, for example, an anti-reflective layer or a barrier layer (notshown).

The term “transparent conductive oxide layer” as used herein refers to asubstantially transparent layer capable of functioning as a frontcurrent collector. In some embodiments, the transparent conductive oxidelayer 112 includes a transparent conductive oxide (TCO). Non-limitingexamples of transparent conductive oxides include cadmium tin oxide(Cd₂SnO₄ or CTO); indium tin oxide (ITO); fluorine-doped tin oxide(SnO:F or FTO); indium-doped cadmium-oxide; doped zinc oxide (ZnO), suchas aluminum-doped zinc-oxide (ZnO:Al or AZO), indium-zinc oxide (IZO),and zinc tin oxide (ZnSnO_(x)); or combinations thereof. Depending onthe specific TCO employed and on its sheet resistance, the thickness ofthe transparent conductive oxide layer 112 may be in a range of fromabout 50 nm to about 600 nm, in one embodiment.

The term “buffer layer” as used herein refers to a layer interposedbetween the transparent conductive oxide layer 112 and the absorberlayer 120, wherein the layer 113 has a higher sheet resistance than thesheet resistance of the transparent conductive oxide layer 112. Thebuffer layer 113 is sometimes referred to in the art as a“high-resistivity transparent conductive oxide layer” or “HRT layer”.

Non-limiting examples of suitable materials for the buffer layer 113include tin dioxide (SnO₂), zinc tin oxide (zinc-stannate (ZTO)),zinc-doped tin oxide (SnO₂:Zn), zinc oxide (ZnO), indium oxide (In₂O₃),or combinations thereof. In some embodiments, the thickness of thebuffer layer 113 is in a range from about 50 nm to about 200 nm.

In some embodiments, as indicated in FIGS. 3-6, the layer stack 110 mayfurther include an interlayer 114 disposed between the buffer layer 113and the absorber layer 120. The interlayer may include a metal species.Non limiting examples of metal species include magnesium, gadolinium,aluminum, beryllium, calcium, barium, strontium, scandium, yttrium,hafnium, cerium, lutetium, lanthanum, or combinations thereof. The term“metal species” as used in this context refers to elemental metal, metalions, or combinations thereof. In some embodiments, the interlayer 114may include a plurality of the metal species. In some embodiments, atleast a portion of the metal species is present in the interlayer 114 inthe form of an elemental metal, a metal alloy, a metal compound, orcombinations thereof. In certain embodiments, the interlayer 114includes magnesium, gadolinium, or combinations thereof.

In some embodiments, the interlayer 114 includes (i) a compoundincluding magnesium and a metal species, wherein the metal speciesincludes tin, indium, titanium, or combinations thereof; or (ii) a metalalloy including magnesium; or (iii) magnesium fluoride; or combinationsthereof. In certain embodiments, the interlayer includes a compoundincluding magnesium, tin, and oxygen. In certain embodiments, theinterlayer includes a compound including magnesium, zinc, tin, andoxygen.

As indicated in FIGS. 3 and 4, in certain embodiments, the absorberlayer 120 is disposed directly in contact with the layer stack 110.However, as further noted earlier, in some embodiments, the photovoltaicdevice 100 may include a discontinuous cadmium sulfide layer interposedbetween the layer stack 110 and the absorber layer 120 (embodiment notshown). In such instances, the coverage of the CdS layer on theunderlying layer (for example, interlayer 114 and the buffer layer 113)is less than about 20 percent. Further, at least a portion of theabsorber layer 120 may contact the layer stack 110 through thediscontinuous portions of the cadmium sulfide layer.

Referring now to FIGS. 5 and 6, in some embodiments, the layer stack 110may further include a window layer 115 disposed between the interlayer114 and the absorber layer 120. The term “window layer” as used hereinrefers to a semiconducting layer that is substantially transparent andforms a heterojunction with an absorber layer 120. Non-limitingexemplary materials for the window layer 115 include cadmium sulfide(CdS), indium III sulfide (In₂S₃), zinc sulfide (ZnS), zinc telluride(ZnTe), zinc selenide (ZnSe), cadmium selenide (CdSe), oxygenatedcadmium sulfide (CdS:O), copper oxide (Cu₂O), zinc oxihydrate (ZnO:H),or combinations thereof. In certain embodiments, the window layer 115includes cadmium sulfide (CdS). In certain embodiments, the window layer115 includes oxygenated cadmium sulfide (CdS:O).

In some embodiments, the absorber layer 120 may function as an absorberlayer in the photovoltaic device 100. The term “absorber layer” as usedherein refers to a semiconducting layer wherein the solar radiation isabsorbed, with a resultant generation of electron-hole pairs. In oneembodiment, the absorber layer 120 includes a p-type semiconductormaterial.

In one embodiment, a photoactive material is used for forming theabsorber layer 120. Suitable photoactive materials include cadmiumtelluride (CdTe), cadmium zinc telluride (CdZnTe), cadmium magnesiumtelluride (CdMgTe), cadmium manganese telluride (CdMnTe), cadmiumtelluride sulfide (CdTeS), zinc telluride (ZnTe), lead telluride (PbTe),mercury cadmium telluride (HgCdTe), lead sulfide (PbS), or combinationsthereof. The above-mentioned photoactive semiconductor materials may beused alone or in combination. Further, these materials may be present inmore than one layer, each layer having different type of photoactivematerial, or having combinations of the materials in separate layers.

As will be appreciated by one of ordinary skill in the art, the absorberlayer 120 as described herein further includes selenium. Accordingly,the absorber layer 120 may further include a combination of one or moreof the aforementioned photoactive materials and selenium, such as, forexample, cadmium selenide telluride, cadmium zinc selenide telluride,zinc selenide telluride, and the like. In certain embodiments, cadmiumtelluride is used for forming the absorber layer 120. In certainembodiments, the absorber layer 120 includes cadmium, tellurium, andselenium.

In some embodiments, the absorber layer 120 may further include sulfur,oxygen, copper, chlorine, lead, zinc, mercury, or combinations thereof.In certain embodiments, the absorber layer 120 may include one or moreof the aforementioned materials, such that the amount of the materialvaries across a thickness of the absorber layer 120. In someembodiments, one or more of the aforementioned materials may be presentin the absorber layer as a dopant. In certain embodiments, the absorberlayer 120 further includes a copper dopant.

In some embodiments, the absorber layer 120, the window layer 115, orboth the layers may contain oxygen. Without being bound by any theory,it is believed that the introduction of oxygen to the window layer 115(e.g., the CdS layer) may result in improved device performance. In someembodiments, the amount of oxygen is less than about 20 atomic percent.In some instances, the amount of oxygen is between about 1 atomicpercent to about 10 atomic percent. In some instances, for example inthe absorber layer 120, the amount of oxygen is less than about 1 atomicpercent. Moreover, the oxygen concentration within the absorber layer120 may be substantially constant or compositionally graded across thethickness of the respective layer.

In some embodiments, the photovoltaic device 100 may further include ap+-type semiconductor layer 130 disposed on the absorber layer 120, asindicated in FIGS. 3-5. The term “p+-type semiconductor layer” as usedherein refers to a semiconductor layer having an excess mobile p-typecarrier or hole density compared to the p-type charge carrier or holedensity in the absorber layer 120. In some embodiments, the p+-typesemiconductor layer has a p-type carrier density in a range greater thanabout 1×10¹⁶ per cubic centimeter. The p+-type semiconductor layer 130may be used as an interface between the absorber layer 120 and the backcontact layer 140, in some embodiments.

In one embodiment, the p+-type semiconductor layer 130 includes aheavily doped p-type material including amorphous Si:H, amorphous SiC:H,crystalline Si, microcrystalline Si:H, microcrystalline SiGe:H,amorphous SiGe:H, amorphous Ge, microcrystalline Ge, GaAs, BaCuSF,BaCuSeF, BaCuTeF, LaCuOS, LaCuOSe, LaCuOTe, LaSrCuOS,LaCuOSe_(0.6)Te_(0.4), BiCuOSe, BiCaCuOSe, PrCuOSe, NdCuOS,Sr₂Cu₂ZnO₂S₂, Sr₂CuGaO₃S, (Zn,Co,Ni)O_(x), or combinations thereof. Inanother embodiment, the p+-type semiconductor layer 130 includes ap+-doped material including zinc telluride, magnesium telluride,manganese telluride, beryllium telluride, mercury telluride, arsenictelluride, antimony telluride, copper telluride, elemental tellurium orcombinations thereof. In some embodiments, the p+-doped material furtherincludes a dopant including copper, gold, nitrogen, phosphorus,antimony, arsenic, silver, bismuth, sulfur, sodium, or combinationsthereof.

In some embodiments, the photovoltaic device 100 further includes a backcontact layer 140, as indicated in FIGS. 3-5. In some embodiments, theback contact layer 140 is disposed directly on the absorber layer 120(embodiment not shown). In some other embodiments, the back contactlayer 140 is disposed on the p+-type semiconductor layer 130 disposed onthe absorber layer 120, as indicated in FIGS. 3-5.

In some embodiments, the back contact layer 140 includes gold, platinum,molybdenum, tungsten, tantalum, titanium, palladium, aluminum, chromium,nickel, silver, graphite, or combinations thereof. The back contactlayer 140 may include a plurality of layers that function together asthe back contact.

In some embodiments, another metal layer (not shown), for example,aluminum, may be disposed on the back contact layer 140 to providelateral conduction to the outside circuit. In certain embodiments, aplurality of metal layers (not shown), for example, aluminum andchromium, may be disposed on the back contact layer 140 to providelateral conduction to the outside circuit. In certain embodiments, theback contact layer 140 may include a layer of carbon, such as, graphitedeposited on the absorber layer 120, followed by one or more layers ofmetal, such as the metals described above.

Referring again to FIG. 6, as indicated, the absorber layer 120 furtherincludes a first region 122 and a second region 124. As furtherillustrated in FIG. 6, the first region 122 is disposed proximate to thelayer stack 110 relative to the second region 124. In some embodiments,the first region 122 is disposed directly in contact with the windowlayer 115. In some embodiments, the first region 122 is disposeddirectly in contact with the buffer layer 113 (embodiment not shown).Further, as discussed earlier, an average atomic concentration ofselenium in the first region 122 is greater than an average atomicconcentration of selenium in the second region 124. In otherembodiments, an average atomic concentration of selenium in the firstregion 122 is lower than an average atomic concentration of selenium inthe second region 124.

In alternative embodiments, as illustrated in FIG. 7, a photovoltaicdevice 200 including a “substrate” configuration is presented. Thephotovoltaic device 200 includes a layer stack 210 and an absorber layer220 disposed on the layer stack. The layer stack 210 includes atransparent conductive oxide layer 212 disposed on the absorber layer,as indicated in FIG. 7. The absorber layer 220 is further disposed on aback contact layer 230, which is disposed on a substrate 240. Asillustrated in FIG. 7, in such embodiments, the solar radiation 10enters from the transparent conductive oxide layer 212 and enters theabsorber layer 220, where the conversion of electromagnetic energy ofincident light (for instance, sunlight) to electron-hole pairs (that is,to free electrical charge) occurs.

In some embodiments, the composition of the layers illustrated in FIG.7, such as, the substrate 240, the transparent conductive oxide layer212, the absorber layer 220, and the back contact layer 230 may have thesame composition as described above in FIG. 5 for the superstrateconfiguration.

A method of making a photovoltaic device is also presented. In someembodiments, the method generally includes providing an absorber layeron a layer stack, wherein the absorber layer includes selenium, andwherein an atomic concentration of selenium varies non-linearly across athickness of the absorber layer. With continued reference to FIGS. 1-6,in some embodiments the method includes providing an absorber layer 120on a layer stack 110.

In some embodiments, as indicated in FIG. 2, the step of providing anabsorber layer 120 includes forming a first region 122 and a secondregion 124 in the absorber layer 120, the first region 122 disposedproximate to the layer stack 110 relative to the second region 124. Asnoted earlier, in some embodiments, an average atomic concentration ofselenium in the first region 122 is greater than an average atomicconcentration of selenium in the second region 124.

The absorber layer 120 may be provided on the layer stack 110 using anysuitable technique. In some embodiments, the step of providing anabsorber layer 120 includes contacting a semiconductor material with aselenium source. The terms “contacting” or “contacted” as used hereinmeans that at least a portion of the semiconductor material is exposedto, such as, in direct physical contact with a suitable selenium sourcein a gas, liquid, or solid phase. In some embodiments, a surface of theabsorber layer may be contacted with the suitable selenium source, forexample using a surface treatment technique. In some other embodiments,the semiconductor material may be contacting with a suitable seleniumsource, for example, using an immersion treatment.

In some embodiments, the semiconductor material includes cadmium.Non-limiting examples of a suitable semiconductor material includecadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), cadmiummagnesium telluride (CdMgTe), cadmium manganese telluride (CdMnTe),cadmium sulfur telluride (CdSTe), zinc telluride (ZnTe), lead telluride(PbTe), lead sulfide (PbS), mercury cadmium telluride (HgCdTe), orcombinations thereof. In certain embodiments, the semiconductor materialincludes cadmium and tellurium.

The term “selenium source” as used herein refers to any materialincluding selenium. Non-limiting examples of a suitable selenium sourceinclude elemental selenium, cadmium selenide, oxides of cadmiumselenide, such as, for example, cadmium selenite (CdSeO₃), hydrogenselenide, organo-metallic selenium, or combinations thereof.

The portion of the semiconductor material contacted with the seleniumsource may depend, in part, on the physical form of the selenium sourceduring the contacting step. In some embodiments, the selenium source isin the form of a solid (for example, a layer), a solution, a suspension,a paste, vapor, or combinations thereof. Thus, by way of example, insome embodiments, for example, the selenium source may be in the form ofa solution, and the method may include soaking at least a portion of thesemiconductor material in the solution.

In some embodiments, the selenium source may be in the form a vapor, andthe method may include depositing the selenium source using a suitablevapor deposition technique. In some embodiments, for example, theabsorber layer 120 may be heat treated in the presence of a seleniumsource (for example, selenium vapor) to introduce selenium into at leasta portion of the absorber layer 120.

In some embodiments, for example, the selenium source may be in the formof a layer, and the method may include depositing a selenium sourcelayer on the semiconductor material, or, alternatively, depositing thesemiconductor material on a layer of the selenium source. In some suchembodiments, the method may further include subjecting the semiconductormaterial to one or more post-processing steps to introduce the seleniuminto the semiconductor material.

Referring now to FIG. 8, in some embodiments, the step of providing anabsorber layer includes (a) disposing a selenium source layer 125 on thelayer stack 110; (b) disposing an absorber layer 120 on the seleniumsource layer 125; and (c) introducing selenium into at least a portionof the absorber layer 120. It should be noted, that the steps (b) and(c) may be performed sequentially or simultaneously.

In some embodiments, the selenium source layer 125 may be disposed onthe layer stack 110 using any suitable deposition technique, such as,for example, sputtering, sublimation, evaporation, or combinationsthereof. The deposition technique may depend, in part, on one or more ofthe selenium source material, the selenium source layer 125 thickness,and the layer stack 110 composition. In certain embodiments, theselenium source layer 125 may include elemental selenium and theselenium source layer 125 may be formed by evaporation. In certainembodiments, the selenium source layer 125 may include cadmium selenide,and the selenium source layer 125 may be formed by sputtering,evaporation, or sublimation.

The selenium source layer may include a single selenium source layer ora plurality of selenium source layers. The selenium source may be thesame or different in the plurality of source layers. In someembodiments, the selenium source layer includes a plurality of seleniumsource layers, such as, for example, a stack of elemental selenium layerand a cadmium selenide layer, or vice versa.

The selenium source layer 125 may have a thickness in a range from about1 nanometer to about 1000 nanometers. In some embodiments, the seleniumsource layer 125 has a thickness in a range from about 10 nanometers toabout 500 nanometers. In some embodiments, the selenium source layer 125has a thickness in a range from about 15 nanometers to about 250nanometers.

As noted, the method further includes disposing an absorber layer 120 onthe selenium source layer 125. In some embodiments, the absorber layer120 may be deposited using a suitable method, such as, close-spacesublimation (CSS), vapor transport deposition (VTD), ion-assistedphysical vapor deposition (IAPVD), radio frequency or pulsed magnetronsputtering (RFS or PMS), chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), or electrochemicaldeposition (ECD).

The method further includes introducing selenium into at least a portionof the absorber layer 120. In some embodiments, the method includesintroducing selenium into at least a portion of the absorber layer 120such that a concentration of selenium varies non-linearly across thethickness of the absorber layer 120.

In some embodiments, at least a portion of selenium is introduced in theabsorber layer 120 simultaneously with the step of disposing theabsorber layer 120. In some embodiments, at least a portion of seleniummay be introduced after the step of disposing the absorber layer 120,for example, during the cadmium chloride treatment step, during thep+-type layer formation step, during the back contact formation step, orcombinations thereof.

In some embodiments, the step of providing an absorber layer 120includes co-depositing a selenium source material and a semiconductormaterial. Suitable non-limiting examples of co-deposition includeco-sputtering, co-sublimation, or combinations thereof. Non-limitingexamples of a suitable selenium source material in such instanceincludes elemental selenium, cadmium selenide, hydrogen selenide,cadmium telluride selenide, or combinations thereof. Thus, by way ofexample, in some embodiments, an absorber layer 120 may be provided bydepositing the semiconductor material in the presence of selenium source(for example, selenium containing vapor or hydrogen selenide vapor).

In some embodiments, the absorber layer 120 may be provided bysputtering from a single target (for example, cadmium selenide telluridetarget) or a plurality of targets (for example, cadmium telluride andcadmium selenide targets). As will be appreciated by one of ordinaryskill in the art, the concentration of selenium in the absorber layer120 may be varied by controlling one or both of target(s) compositionand sputtering conditions.

As noted earlier, the photovoltaic device 100 and the layer stack 110may further include one or more additional layers, for example, asupport 111, a transparent conductive oxide layer 112, a buffer layer113, an interlayer 114, a p+-type semiconductor layer 130, and a backcontact layer 140, as depicted in FIGS. 3-5.

As understood by a person skilled in the art, the sequence of disposingthe three layers or the whole device may depend on a desirableconfiguration, for example, “substrate” or “superstrate” configurationof the device.

In certain embodiments, a method for making a photovoltaic 100 insuperstrate configuration is described. Referring now to FIGS. 3-6, insome embodiments, the method further includes disposing the transparentconductive oxide layer 112 on a support 111. The transparent conductiveoxide layer 112 is disposed on the support 111 by any suitabletechnique, such as sputtering, chemical vapor deposition, spin coating,spray coating, or dip coating. Referring again to FIGS. 3-6, in someembodiments, a buffer layer 113 may be deposited on the transparentconductive oxide layer 112 using sputtering. The method may furtherincluding disposing an interlayer 114 on the buffer layer 113 to form alayer stack 110, as indicated in FIG. 4.

The method may further include disposing a window layer 115 on theinterlayer 114 to form a layer stack 110, as indicated in FIGS. 5 and 6.Non-limiting examples of the deposition methods for the window layer 115include one or more of close-space sublimation (CSS), vapor transportdeposition (VTD), sputtering (for example, direct current pulsesputtering (DCP), electro-chemical deposition (ECD), and chemical bathdeposition (CBD).

The method further includes providing an absorber layer 120 on the layerstack 110, as described in detail earlier. In some embodiments, a seriesof post-forming treatments may be further applied to the exposed surfaceof the absorber layer 120. These treatments may tailor the functionalityof the absorber layer 120 and prepare its surface for subsequentadhesion to the back contact layer(s) 140. For example, the absorberlayer 120 may be annealed at elevated temperatures for a sufficient timeto create a quality p-type layer. Further, the absorber layer 120 may betreated with a passivating agent (e.g., cadmium chloride) and atellurium-enriching agent (for example, iodine or an iodide) to form atellurium-rich region in the absorber layer 120. Additionally, coppermay be added to absorber layer 120 in order to obtain a low-resistanceelectrical contact between the absorber layer 120 and a back contactlayer(s) 140.

Referring again to FIGS. 3-6, a p+-type semiconducting layer 130 may befurther disposed on the absorber layer 120 by depositing a p+-typematerial using any suitable technique, for example PECVD or sputtering.In an alternate embodiment, as mentioned earlier, a p+-typesemiconductor region may be formed in the absorber layer 120 bychemically treating the absorber layer 120 to increase the carrierdensity on the back-side (side in contact with the metal layer andopposite to the window layer) of the absorber layer 120 (for example,using iodine and copper). In some embodiments, a back contact layer 140,for example, a graphite layer may be deposited on the p+-typesemiconductor layer 130, or directly on the absorber layer 120(embodiment not shown). A plurality of metal layers may be furtherdeposited on the back contact layer 140.

One or more of the absorber layer 120, the back contact layer 140, orthe p+-type layer 130 (optional) may be further heated or subsequentlytreated (for example, annealed) after deposition to manufacture thephotovoltaic device 100.

In some embodiments, other components (not shown) may be included in theexemplary photovoltaic device 100, such as, buss bars, external wiring,laser etches, etc. For example, when the device 100 forms a photovoltaiccell of a photovoltaic module, a plurality of photovoltaic cells may beconnected in series in order to achieve a desired voltage, such asthrough an electrical wiring connection. Each end of the seriesconnected cells may be attached to a suitable conductor such as a wireor bus bar, to direct the generated current to convenient locations forconnection to a device or other system using the generated current. Insome embodiments, a laser may be used to scribe the deposited layers ofthe photovoltaic device 100 to divide the device into a plurality ofseries connected cells.

EXAMPLES Example 1: Method of Fabricating a Photovoltaic Device with aNon-Linear Gradient Profile

A cadmium telluride photovoltaic device was made by depositing severallayers on a cadmium tin oxide (CTO) transparent conductive oxide(TCO)-coated substrate. The substrate was a 1.4 millimeters thick PVN++glass, which was coated with a CTO transparent conductive oxide layerand a thin high resistance transparent zinc tin oxide (ZTO) bufferlayer. A magnesium-containing capping layer was then deposited on theZTO buffer layer to form an interlayer. The window layer (approximately40 nanometers thick) containing cadmium sulfide (CdS:O, withapproximately 5 molar % oxygen in the CdS layer) was then deposited onthe interlayer by DC sputtering and then annealed at an elevatedtemperature. An approximately 500 nm thick Cd(Te,Se) film was thendeposited by close space sublimation from a source material with aSe/(Se+Te) ratio of approximately 40%. The pressure was fixed atapproximately 15 Torr with a small amount of oxygen in the He backgroundgas. After deposition, the stack was treated with CdCl₂ and then bakedat temperature greater than 400° C. Following the bake excess CdCl₂ wasremoved. Approximately 3.5 microns CdTe film was then deposited by closespace sublimation in the presence of about 1 Torr of O₂. After thesecond deposition, a second CdCl₂ treatment and subsequent bake followedby removal of excess CdCl₂ was performed before forming a back contact.

The Se deposition profile in the device was measured using dynamicsecondary ion mass spectroscopy (DSIMS) performed. Prior to themeasurement, the samples were polished to reduce the effects of surfaceroughness. The results for Se ion concentration (in atoms/cm³) are shownin FIG. 9. The peak of the Se concentration is near the location windowand buffer layers. The depth axis is the distance in microns from thepolished edge of the sample. Since the polishing procedure removes someamount of CdTe, the total thickness of the alloy layer is less than thethickness of the Cd(Se)Te alloy layer of the original solar cell.

To assess the non-linear nature of the distribution of the Se within theabsorber layer the data was filtered to remove points after the peak ofthe Se distribution in the in data. The data was then plotted on alog-log plot. The data is shown in FIG. 10. Two functions were fitted onthe log-log plot: one a linear fit which a slope of 1.27, which isindicative of super-linear distribution. Since the overall fit qualitywas poor, the log-log data was also fit to an exponentially risingfunction, which gave a significantly better fit indicating that themeasured Se distribution is highly non-linear.

Examples 2-4 Simulation Tests for Different Non-Linear Se ConcentrationProfiles in a CdTe Layer

To illustrate some of the non-linear profiles, simulations were carriedout using the one-dimensional solar cell simulation program SCAPSv.3.2.01 (M. Burgelman, P. Nollet and S. Degrave, “Modellingpolycrystalline semiconductor solar cells”, Thin Solid Films 361-362(2000), pp. 527-532) The program numerically solves the Poisson andcontinuity equations for electrons and holes in a single dimension todetermine the band-diagram of the device and its response toillumination, voltage bias, and temperature. Performance calculationswere made using simulated IV sweeps in the simulation under illuminationby the AM1.5G spectrum at 100 mW/cm² of intensity and 300K, also knownas Standard Test Conditions (STC). The model parameters for CdTe anddevice design were set according to the parameters given by Gloeckler etal. for CdTe solar cells. (M. Gloeckler, A. Fahrenbruch and J. Sites,“Numerical modeling of CIGS and CdTe solar cells: setting the baseline”,Proc. 3rd World Conference on Photovoltaic Energy Conversion (Osaka,Japan, may 2003), pp. 491-494, WCPEC-3, Osaka (2003)), except that theCdTe absorber layer thickness was increased to 4.5 microns and thenature of the deep trap in the CdTe absorber layers was changed from‘donor’ to neutral. The CdSe parameters were set to have the same valuesas the CdTe parameters, except that the deep trap density in the CdSe isa factor of ten lower and the band gap is 1.7 eV. A model for thevariation in the properties of the alloy material CdTe_(1-x)Se_(x) as afunction of x, the fraction Se substitution, was constructed. The modelassumes that the Eg of the CdTe is equal to 1.5 eV, the gap of the CdSeis equal to 1.7 and a bowing parameter, b=0.8. The band gap of the alloyis given by:E _(g,alloy) =xE _(g,CdSe)+(1−x)E _(g,CdTe) −bx(1−x).The other material properties, such as carrier mobilities and dielectricconstant values were assumed to be independent of alloy composition andthe deep donor concentration varied linearly between the CdTe and CdSevalues as function of x.

In Example 2, simulation was conducted using the measured DSIMS Seprofile as input. The measured DSIMS profile was fit to a bi-exponentialdecay profile and the parameters from the fit used to calculate a Seconcentration profile throughout the 4.5 micron thickness of theabsorber layer.

In Example 3, an exponential Se concentration profile was assumed,rising from about x=0.006 in the back to 0.2 in the front. The totalamount of Se in the device was about 4.4 times that of the devicedescribed in Example 1.

In Example 4, a top-hat Se concentration profile was assumed. In theparticular top-hat profile, x=0 from the back of the device until about0.4 microns from the front interface, whereupon it rises. From thispoint, x=0.4 until the front of the absorber layer is reached. The totalamount of Se in the device was about 3 times that of the devicedescribed in Example 1.

Comparative Example 1 Simulation Test for a Conventional CdS/CdTePhotovoltaic Device

For this simulation, the device had no Se and used the inputs asspecified by Gloeckler except for the modifications noted previously.The calculated performance metrics of this model cell (efficiency, Voc,Jsc, and fill factor (FF)) were used as the reference levels for theother examples and their respective performance metrics were normalizedto this baseline case.

Comparative Example 2 Simulation Test for a Linear Se ConcentrationProfile in CdTe Layer

For this simulation, a linear Se concentration profile was used assumingthe same total amount of Se as determined via the DSIMS profile. In thiscalculation, a linear gradient in Se concentration was input into thedevice model. The value of x was set to 0 at the back contact and toabout 0.025 in front.

Comparative Example 3 Simulation Test for a Constant Se ConcentrationProfile in CdTe Layer

For this simulation, a constant Se concentration profile with x=0.4 wasassumed throughout the absorbing layer of the device. The total amountof Se in the device was about 32 times that of the device described inExample 1.

Comparative Example 4 Simulation Test for a Linear Se ConcentrationProfile in CdTe Layer

For this simulation, the Se concentration profile was assumed to be alinear ramp starting from x=0 at the back contact and rising to x=0.4 atthe front of the device. The total amount of Se in the device is about16 times that of the device described in Example 1.

The performance metrics of Examples 2-4 and Comparative Examples 2-4relative to the baseline cell of Comparative Example 1 are reported inTable 1. FIG. 11 shows the Se concentration profile as a function ofCdTe thickness for Comparative Examples 2-4 and Examples 2-4.

As illustrated in Table 1, the device performance parameters showedimprovement for the devices with a non-linear graded CdTeSe layer(Examples 2-4) when compared to the device without a CdTeSe layer(Comparative Example 1). For the same amount of Se, the deviceperformance parameters further showed improvement for the devices with anon-linear graded CdTeSe layer (Example 2) when compared to the devicewith a linear gradient of Se in CdTeSe layer (Comparative Example 2).Both the ‘exponential’ and ‘top-hat’ non-linear Se concentrationprofiles (Examples 3-4) demonstrated superior efficiency to cells thathad either constant or linearly graded Se concentration profiles(Comparative Examples 3 and 4), despite have a much lower total amountof Se present in the layer.

It should be noted that while the profiles in the example set areprimarily confined the front, some degree of shifting of the Se profilemay lead to improvement in overall device performance, particularly ifthe doping profiles or the energy levels of the front and back contactsare adjusted. In such cases it is possible that optimal position of thepeak of the Se is not at the front interface next to the buffer layer.

TABLE 1 Simulation results for performance parameters for different Seconcentration profiles in CdTe Effi- Relative ciency Amount Peak Example(%) Voc Jsc FF of Se Peak Location Comparative 1.000 1.000 1.000 1.0000.0 0.00 none Example 1 Comparative 1.004 0.991 1.014 0.998 1.0 0.03front Example 2 Comparative 1.060 0.893 1.186 1.002 32.0 0.40 noneExample 3 Comparative 1.041 0.924 1.169 0.963 16.0 0.40 front Example 4Example 2 1.033 0.999 1.028 1.006 1.0 0.25 front Example 3 1.102 0.9981.100 1.004 4.4 0.20 front Example 4 1.102 0.998 1.100 1.004 3.0 0.40front 0.4 microns

The appended claims are intended to claim the invention as broadly as ithas been conceived and the examples herein presented are illustrative ofselected embodiments from a manifold of all possible embodiments.Accordingly, it is the Applicants' intention that the appended claimsare not to be limited by the choice of examples utilized to illustratefeatures of the present invention. As used in the claims, the word“comprises” and its grammatical variants logically also subtend andinclude phrases of varying and differing extent such as for example, butnot limited thereto, “consisting essentially of” and “consisting of.”Where necessary, ranges have been supplied; those ranges are inclusiveof all sub-ranges there between. It is to be expected that variations inthese ranges will suggest themselves to a practitioner having ordinaryskill in the art and where not already dedicated to the public, thosevariations should where possible be construed to be covered by theappended claims. It is also anticipated that advances in science andtechnology will make equivalents and substitutions possible that are notnow contemplated by reason of the imprecision of language and thesevariations should also be construed where possible to be covered by theappended claims.

The invention claimed is:
 1. A photovoltaic device, comprising: a layerstack; and an absorber layer disposed on the layer stack, the absorberlayer having a light incident side proximate to the layer stack and aback side opposite the light incident side, wherein: the absorber layercomprises a CdTe_(1-x)Se_(x) alloy, where x is greater than 0 and lessthan 0.4 throughout the absorber layer; an atomic concentration ofselenium varies across a thickness of the absorber layer; the atomicconcentration of selenium in the absorber layer is greater proximate tothe light incident side relative to the back side of the absorber layer;the absorber layer has a band gap which varies across the thickness ofthe absorber layer; and the band gap is lower proximate to the lightincident side relative to the back side of the absorber layer.
 2. Thephotovoltaic device of claim 1, wherein the band gap of the absorberlayer is less than or equal to about 1.5 eV.
 3. The photovoltaic deviceof claim 1, wherein the band gap decreases across the thickness of theabsorber layer from the back side to the light incident side.
 4. Thephotovoltaic device of claim 1, wherein band gap properties of theCdTe_(1-x)Se_(x) alloy vary as a function of x, and the band gap of thealloy is:Eg, _(alloy) =xEg, _(CdSe)+(1−x)Eg, _(CdTe) −bx(1−x) wherein a bowingparameter, b=0.8.
 5. The photovoltaic device of claim 1, wherein theatomic concentration of selenium in the absorber layer varies from aboutx=0.2, proximate to the light incident side, to about x=0.006, proximateto the back side of the absorber layer.
 6. The photovoltaic device ofclaim 1, wherein the layer stack comprises: a transparent conductivelayer disposed on a support; and a buffer layer disposed between thetransparent conductive layer and the absorber layer.
 7. The photovoltaicdevice of claim 1, wherein the atomic concentration of selenium in theabsorber layer has a profile comprising an exponential profile, atop-hat profile, a step-change profile, a saw-tooth profile, asquare-wave profile, a power law profile, or combinations thereof. 8.The photovoltaic device of claim 1, wherein a peak of a Se/(Se+Te) ratioof the absorber layer is located at a front interface of the absorberlayer, proximate to the layer stack, and, wherein the peak of theSe/(Se+Te) ratio of the absorber layer is less than 0.40.
 9. Thephotovoltaic device of claim 1, wherein at least a portion of theabsorber layer comprising cadmium, selenium, and tellurium is a ternarycompound or a quaternary compound.
 10. The photovoltaic device of claim1, wherein the absorber layer further comprises sulfur, oxygen, copper,chlorine, lead, zinc, mercury, or combinations thereof.
 11. Aphotovoltaic device, comprising: a layer stack comprising a transparentconductive oxide; and an absorber layer disposed on the layer stack, theabsorber layer having a light incident side proximate to the layer stackand a back side opposite the light incident side, the absorber layercomprising: a CdTe_(1-x)Se_(x) alloy, where 0<x<0.4, wherein band gapproperties of the CdTe_(1-x)Se_(x) alloy vary as a function of x, andthe band gap of the alloy is:Eg, _(alloy) =xEg, _(CdSe)+(1−x)Eg, _(CdTe) −bx(1−x) where a bowingparameter, b=0.8; wherein an atomic concentration of selenium variesacross a thickness of the absorber layer; the atomic concentration ofselenium in the absorber layer is greater proximate to the lightincident side relative to the back side of the absorber layer; theabsorber layer has a band gap which varies across the thickness of theabsorber layer; and the band gap is lower proximate to the lightincident side relative to the back side of the absorber layer.
 12. Thephotovoltaic device of claim 11, wherein the band gap of the absorberlayer is less than or equal to about 1.5 eV.
 13. The photovoltaic deviceof claim 11, comprising a first region and a second region, wherein: thefirst region is disposed proximate to the layer stack relative to thesecond region, the first region is disposed at a front interface of theabsorber layer, the second region is disposed at a back interface of theabsorber layer, and wherein band gap in the first region is lower thanin the second region.
 14. The photovoltaic device of claim 13, wherein,the first region has a thickness between 100 nanometers to 3000nanometers, the second region has a thickness between 100 nanometers to3000 nanometers, a ratio of an average atomic concentration of seleniumin the first region to an average atomic concentration of selenium inthe second region is greater than 2, and the average atomicconcentration of selenium in the absorber layer is in a range from 0.01atomic percent to 25 atomic percent of the absorber layer.
 15. A methodof making a photovoltaic device, comprising: providing an absorber layeron a layer stack, wherein the absorber layer comprises selenium, andwherein an atomic concentration of selenium varies non-linearly across athickness of the absorber layer, whereby: the absorber layer comprises aCdTe_(1-x)Se_(x) alloy, where x is greater than 0 and less than 0.4throughout the absorber layer; the absorber layer has a light incidentside proximate to the layer stack and a back side opposite the lightincident side, the atomic concentration of selenium in the absorberlayer is greater proximate to the light incident side relative to theback side of the absorber layer; the absorber layer has a band gap whichvaries across the thickness of the absorber layer; and the band gap islower proximate to the light incident side relative to the back side ofthe absorber layer.
 16. The method of claim 15, wherein the atomicconcentration of selenium varies non-monotonically across a thickness ofthe absorber layer.
 17. The method of claim 15, wherein the step ofproviding an absorber layer comprises: disposing a selenium source layeron the layer stack, wherein the selenium source comprises elementalselenium, cadmium selenide, hydrogen selenide, organo-metallic selenium,or combinations thereof; disposing a semiconductor material on theselenium source layer, wherein the semiconductor material comprisescadmium and tellurium; and introducing selenium from the selenium sourcelayer into at least a portion of the absorber layer.
 18. The method ofclaim 17, wherein the selenium source layer has an average thickness ina range from about 1 nanometer to about 1000 nanometers.
 19. The methodof claim 16, wherein the step of providing an absorber layer comprisesco-depositing a selenium source material and a semiconductor material.20. The photovoltaic device of claim 1, wherein the absorber layer isp-type.